Induction electric furnace



March 9, 1937. F NQRTHRUP 2,073,597

INDUCTION ELECTRIC FURNACE Original Filed May 22, 1930 2 Sheets-Sheet 1 if if March 9, 1937. E. F. NORTHRUP 2,073,597

INDUCTION ELECTRIC FURNACE Original Filed May 22, 193-2) 2 Sheets-Sheet 2 TIME IN MINUTES J I 64", 2% i .m z M J ,5

Patented Mar. 9, 1937 UNITED STATES PATENT OFFICE INDUCTION ELECTRIC FURNACE Application May 22, 1930, Serial No. 454,581 Renewed September 3, 1936 17 Claims.

My invention relates to improvements in the structure of electric induction furnaces.

A purpose of my invention is to shield the magnetic parts of an induction electric furnace from excessive heating due to stray flux from the inductor coil.

A further purpose is to permit the use of magnetic steel structure in an induction electric furnace in placeof more expensive non-magnetic alloys.

. A further purposeis to conduct the heat from the stray field developed in the structure of an induction, electric furnace to a .distant point where the heat is dissipated.

A further purpose is to make it unnecessary to use compensator coils or magnetic return circuits about the inductor of a furnace and yet to not limit the power input to the furnace.

A further purpose is to increase the metallic structural support of an inductor coil without increasing the coil losses.

A further purpose is to consume the energy of the stray field about an inductor coil in inducing current in a non-magnetic shield, thus preventing the lines of force from proceeding further.

Further. purposes appear in the specification and in the claims.

In the drawings I illustrate two only of the desirable forms of my invention, choosing them from the standpoint of their efficiency in carrying out my purpose and of the ease with which they may be illustrated.

Figure 1 is a top plan view of an induction furnace to which my invention has been applied. The cover. of the-furnace has been removed for more convenient illustration of the inductor structure, and certain non-essential details have been omitted.

Figure 2 is a side elevation of the furnace of 40 Figure 1.

Figure 3 is a fragmentary section taken on the line 33 of Figure 2, omitting structure in the background.

Figure 4 is'a temperature-time curve of a test 45 showing an example of the use which m'yinvention serves.

Figure '5 corresponds generally to Figure 3,- but shows a variant application of my invention. In the drawings similar numerals refer to like 50 parts throughout.

A great deal of difficulty has been met with in the manufacture and use of induction electric furnaces of the careless type because of the interference of themagnetic return circuit of the. 55 furnace with the use of normal steel or iron for the frame parts, of the furnace. This difficulty has'been greatest with the larger furnaces because, with the increased weight, the necessityfor strong frame members and the size of these members are increased. I

Attempts have been made to avoid the difficulty in various ways, as by additional spacing of the frame parts from. the furnace, the provision of a magnetic return circuit for the furnace, which is independent of the frame of the furnace by the use of an additional inductor coil (compensator coil) outside of the main inductor coil to take care of stray field, and by various other means. All of these ofier difficulties in some constructions.

Compensator coils are disclosedin prior patents, for example my Patent No. 1,744,983, granted January 28, 1930, for Inductor furnace, the patent to Jones, No. 1,608,560, granted November 30, 1926, for Coil'system, and that to Banning, No. 1,667,715, granted May 1, 1928, for Inductive unit and the like.

Furnace designers have therefore favored the use of non-magnetic alloys for the structural parts, particularly in the case of the larger in.- duction furnaces. Alloys of'the brass type are lacking in the requisite tensile strength, Ferrous non-magnetic alloys, as for example ascoloy (an iron alloy containing 8% of nickel and 18% of chromium, generally classed as a stainless steel), are quite expensive, and I therefore aim to eliminate such materials or to'reduce them to a minimum.

My invention aims to absorb stray lines of force from the furnace with a'minimum of inter ,ference with the construction of the furnace frame and with a maximum cooling efiect to dissipate the heat due to the absorption.

Byv my invention I place a non-magnetic shield between the inductor coil and the magnetic parts of the furnace. By the introduction of such aatively lowheat conductivities as compared with non-magnetic materials such as copper. There Steel and similar magnetic materials have rel- 2 fore by using a shield having a high heat eon ductivity, the heat due to BI -energy losses and toconduction from the crucible and charge will be rapidly distributed to points remote from the furnace pool, instead of remaining concentrated coil, my shield should preferably be longer than the coil to cut olf all stray field from the mag- .netic parts.

As seen in Figures 1 and 2, the inductor coil I0 surrounds the crucible containing the charge.

The inductor coil is supported by struts I2 from the outer'casing of the furnace. Current issupplied to the inductor coil at the points 13 and l3 through leads l4, l4 from a generatonnot ,shown. The power factor may be desirably corrected by the condenser I4 In the outer casing of the furnace heat insulation l5 completely surrounds the inductor coil. One material which hasbeen found satisfactory because of its heat insulating properties and strength is asbestos board.

The sides of the furnace nearest the magnetic supports are covered'with non-magnetic shields 16, preferably of copper. For these should extend the full distance to the edges so that they may be held by the angle strip I1 and I8, pi eferably of brass or similar. non-magnetic material. Beyond the shield and the angle plates diagonal 35 supporting strips 'l9' of magnetic material extend fro m the top of the furnace atthe frontto the bottom at the'rear. At the upper end-of the supporting strips trunnion plates supporting trunnions 2| are secured by bolts, 22 extending 40 through the diagonal supporting strips and in some cases through the shield andheat insulating material. 5

Q At the base of the diagonal supporting strips, bolts 23 secure them to flanges on an angle 45 strip 25 extending along the bottom back edge of the furnace.

bottom on each side for engagemeht by hooks on fa crane used to tilt thefurnace about one of .the

sets of trunnionsl .At the front of the furnace a 5() eovrner strip 21' unites the ends of the angle strips As best seen in Figure 3, the iron-magnetic shield l6 covers the'ent' e fac'e of the furnace 7 inside of the magnetic-diagonal supporting mem- 55 ber 19 and trunnion plate 20.

The distance between the frame parts of .the

furnace and the inductor coil is selected soas to give ample. room for an intermediate return magnetic circuit and so' that the linesof force tending to heat the metal of the frame'w'ill be a relatively small and stray part of the magnetic field. Notwithstahding this, the low reluctance of the magneticpath "through magnetic frame parts'so reduces the reluctance ofthe return cir- 5 cuitf thatan excessive part of the magnetic re-' turn lcrowds' through 'these frame 'p'arts, excesslvely-heating the frame'jn the absence of some means of protection.

The magnetic lines which tend to pass beyond 7 the shield, l6 are, 50min pass into, if: not through, this shieldf In their travelwithin the 'shield they setup electric currents in the metal of the shield, frittering away their magnetomotive force in the passages 75 -The depthto whichthewlines of force extendof low heat concurrent within the shield.

structural reasons .A clevis 26 is also placed atthelocated in a stronger into the nonmagnetic electrically conducting 'shield restricts the depth at which the electric currents are induced within the shield and the extent of current induction there varies with the strength of the magnetic force which progres-' sively decreases within the shield.

. A thickness of shield may be selected for any. given frequency which will whollyor efiec'tively; for all practical purposes, absorb the lines of force, 50 that no lines of force, ornot enough toproduce undesirable effects, pass through the shield. Here,as in' the case of the induction of currentwithin any secondary, complete absorption of the magnetomotive force of the stray field within the'shield represents a thickness of shield corresponding approximately to three times the. eflectlve depth of penetrationf, often called simply the depth/of penetration, of the For example, for a frequency of one thousand cycles-the thickness ofthe shield l6 should be about one-quarter inch, preferably of copper. For a discussion of (effective depth of penetration .see my Patents No. 1,694,781, granted December 11,.1928,-for Induction electric furnace, and No. 1,694,792, granted December 11,1928, for

High frequency ,induction furnace, and Stein-j metz, Theory and Calculation of Transient Electric Phenomena arid Oscillations", Chapters VI and VII, and especially page 383.

Material on the inside of the shield l6 should desirably be non-magnetic. Thus in Figure 3, the washers 28 may be of magnetic material, but the bolt'22, the nut 29 and the washer 30 inside the shield l6 should preferably be of ascoloy.

However, as the direction of v fluxnear the.

bolt is vertical, and the vertical projected areaof the bolt which will interceptfiux is very small,,

I may form my b01 tc of magnetic material without very high heating init, although high heating would occur in large structural members ifthey'were similarly'exposed. Besides perform; ing th'e function of a shield, the non-magnetic plate l6 adds additional. strength to the structure, particularly whereas shown in my drawings', it extends under the angle strips to be engaged" by them and'united to them if desired.

Any predetermined approach to complete ab- 7 \sorption of the within the shield maybe effected which proves desirable under the conditions of intended. use.

Tests of this invention show that a copper;

shield substantially absorbing all of the stray field does not heat {anywhere nearly as inucli as;

the absence of the shield, in .such an arrangementas is shown inFigures 1, 2 and 3, for example. This is the case notwithstanding that the magnetic frame parts of the furnaceheat in stray fieldmagnetomotive force the shield is closer to the coilfand therefore is x v 1 part offfthe return circuit magnetic'field than the magnetic frame parts of the furnace. The difference in heating seems out of proportion to such aswould be anticipated fromthe better distribution of heat .within the shield on account of thef-higher thermal! con- ,ductivity of the copper and theconsequent hen ter heat-distribution and heat dissipation from the larger copper surface .Figure 4 illustrates the result ofa test made upon-a furnace in which'theimagnetic structural members on one side were shielded by copper as here shown, and those on urnace were not-provided with shielding. The

distance from thecen'terof thecoil to the mag the other side of the structural pieces,

shown by the curve B, while the protected structural diagonal l9 increased in temperature along the curve C. 7

On the other side of the furnace, the unprotected structural diagonal increased in temperature as indicated vby the curve D. Thus after one hundred sixty minutes the room temperature was 21 C., the temperature of the shield and of the protected diagonal was approximately 26 C., while the temperature of the unprotected diagonal member rose to 92. All curves are exterpolated beyond one hundred sixty minutes.

It will of course be understood that the test here recorded is merely illustrative and is not intended to limit the invention to any 'conditions of use.

This very considerable reduction in temperature' is more remarkable because the copper is located closer to the inductor coil, and there-: fore within a stronger field than the magnetlz able material. In so far as additional flux might pass through the magnetizable frame parts, be-' cause theyofier a path of lower reluctance for the return of the field, no change has been made in the position of these magnetizable parts,

In Figure 5, I show by way of illustration a less desirable form of my invention. In this form I omit the non-magnetic shield l6 and instead place a non-magnetic coveringiB' on the exposed surfaces of the magnetic supports id and- 29. For example, the diagonal support it) may be formedfrom a copper and a steel billet which are placed together while highly heated and rolled, to weld the copper to the surface of the that it is not necessary thatthe non-magnetic shield be a separate member from the magnetic In this form the Washers 28' will preferably be of ascoloy since they are inside the shield.

The construction of Figures 1, 2 and 3 is considerably better" than that of Figure 5 from the standpoint both of more nearly total absorption of the lines of force and from the collateral standpoint that the larger area and more extended physical distribution of the copper in Figures 1-3, ofiers not only a larger area within which the stray field may be intercepted and dissipated but also a much larger area for convection, conduction and radiation of heat from the shield.

However, tests of the construction of Figures 1, 2 and 3 have shown'a reduction of temperature in the copper when used below that of the such in so far as they fall within the reasonable spirit and scope of my'invention.

Having thus described my invention, what I claim. as new and desire to, secure by Letters Patent is:

1. The method. of eliminating stray field heating in magnetic parts of an induction electric furnace, which consists in intercepting the stray magnetic flux by a non-magnetic metal in solid condition located between the point of emanation and the magnetic parts and in converting the flux into heat within this non-magnetic metal.

2. The methodof eliminating stray field heating in magnetic parts of an induction electric furnace, which consists in providing a solid electrical path for eddy-current generation between the furnace inductor and the magnetic parts and extending only part of the way around the furnace inductor, which path is opaque to passage of flux through it, in transforming the flux into induced current and in conducting the heat away from the point of eddy-current heat generation.

.3. In an induction electric furnace, a furnace inductor coll, magnetic frame parts for the furnace at a distant point from the coil and a shield of non-magnetic, highly conductive solid material located between the coil and the frame parts in the line of travel of stray flux and of sumcient thickness to transform into heat substantially all of the stray flux passing into it.

4. In an induction electric furnace, an inductor coil, supporting structure for the inductor coil containing magnetic parts and a non-magnetic solid metallic shield betweenthe inductor coil and the magnetic parts to cut off the magnetic flux from the parts, the shield being substantially discontinuous about the inductor,

5. In an electric induction furnace, an inductor coil, supporting structure for the inductor coil containing magnetic parts, and a non-magnetic solid metallic shield between the inductor coil and the magnetic parts and closer to the inductor at points located between the inductor coil and parts than at the extremities of the shield.

6. In an electric induction furnace, an inductor coil, supporting structure for the inductor coll containing magnetic parts and a non-magnetic solid metallic shield having a thickness greater than the effective depth of penetration of the magnetic flux placed between the inductor coil and the magnetic parts.

"7. In an electric induction furnace, an inductor coil, supporting structure for the inductor coil containing magnetic parts and a non-magnetic solid metallic shieldof substantially greater coil containing magnetic parts and a non-magnetic solid metallic shield of a thickness greater .solid metallic shield of high electrical and heat conductivities between the inductor coil and the magnetic parts.

10. In an electric induction furnace, an inductor coil, supporting structure for the inductor coil containing magnetic parts and a discontinuous non-magnetic solid metallic shield of high xmetallic solid shield of high ele electrical conductivity between theinductor coil and the magnetic parts.

tinuous, non-magnetic solid metallic shield of high electrical and heat conductivities betweenthe inductor coil and the magnetic parts.

12. In an electric induction furnace, an inductor coil, supporting structure for the inductor coil containing magnetic parts and a fiat non-magnetic metallic'plate between the inductor coil and the magnetic parts.

13 In .an electric induction furnace, an inductor coil, supporting structure for the inductor coil containing magnetic'parts'and a flat copper plate between the inductor coil and the magnetic parts.

14. In an electric induction furnace, an inductor coil, supporting structure for the inductor coil containing magnetic parts and g. non-magnetic trical and heat conductivities between the inductor coil and the magnetic parts and extending uninterruptedly to an angular extent measured about .-the furnace substantially-as great as the angular extent of the magnetic parts to be protected.

15. An inductor furnace comprising a crucible,

aninducing windin surrounding said crucible, a metallic framework outside'of said winding to be protected from the magnetic efiects of the current in said winding and a copper shield located between said winding and framework and continuous and imperforate throughout the horizontal angular extent of said framework.

16. An inductor furnace comprising a crucible,

an inducing winding surrounding the said crucible', a metallic framework outside the said winding and a solid copper shield interposed between said winding and said framework and shielding said framework throughout the entire horizontal winding as great as the angular extent 'of the supporting framework and interposed between said winding and said framework, sai'd'shield having a thickness at least as great as its depth of magnetic penetration.

EDWIN NoR'mnfiP. 

